1. Field of the Invention
The present invention relates to a method for producing fine particles of barium ferrite useful as a magnetic recording material. More particularly, it relates to a method for producing fine particles of barium ferrite, which comprises reacting an aqueous solution comprising an iron compound, a barium compound and an alkaline substance under a subcritical to supercritical condition for water by means of a flow type reactor.
2. Discussion of Background
In recent years, to meet the requirements for high density magnetic recording, various attempts have been made to develop vertical magnetic recording media by barium ferrite. As a method for producing such barium ferrite, a flux method and a hydrothermal synthetic method are mainly known.
For example, Japanese Unexamined Patent Publication No. 60002/1981 discloses a method wherein an alkali and an alkali carbonate are mixed to an aqueous solution containing an iron (II) salt and a barium salt with stirring to obtain a coprecipitate of ferric hydroxide and barium carbonate, and then the coprecipitate is collected by filtration, washed with water and dried, and then subjected to heat treatment to obtain barium ferrite particles. This method has a merit in that the process is simple and less costly, but it has problems such that it is difficult to obtain the monodisperse particles, and the coprecipitate is poor in the filtration efficiency.
Japanese Unexamined Patent Publication No. 160328/1981 discloses a method wherein an alkaline solution having an iron (II) salt and a barium salt dissolved therein, is treated at a temperature of from 150.degree. to 250.degree. C. in an autoclave to form a precipitate of a barium ferrite precursor, which is then calcined at a temperature of at least 800.degree. C. to obtain barium ferrite powder. By this method, the above-mentioned problem relating to the filtration efficiency of the coprecipitate, can be overcome. However, this method requires a long reaction time, and is a batch method for hydrothermal treatment by means of an autoclave. Therefore, the method has poor productivity and has a problem of high cost in a large scale.
Japanese Unexamined Patent Publication No. 67904/1981 discloses a method wherein components forming barium ferrite, such as barium oxide and iron oxide, and components forming flux such as boron oxide, are mixed and melted, and the melt is rapidly cooled and solidified, and then subjected to heat treatment to precipitate barium ferrite in the flux, and the obtained fine powder is then treated with dilute acid to dissolve and remove the flux components and thereby to extract barium ferrite particles, which are then washed with water and dried. By this method, a fine powder having a sharp particle size distribution can be obtained. However, a high temperature is required to melt the starting materials, and reheating treatment is required. Thus, the process for the production tends to be complicated, and the productivity will be very low.
The present inventor have previously filed a patent application i.e. Japanese Unexamined Patent Publication No. 50105/1992 for a method for continuously producing fine particle of metal oxide by reacting an aqueous solution of a metal salt at a temperature around subcritical or supercritical temperature for water by means of a flow type reactor. As a result of various studies for applying this method to the production of barium ferrite, they have now found a method for selectively producing fine particles of barium ferrite of hexagonal crystal (BaO.6Fe.sub.2 O.sub.3) by overcoming the above-mentioned conventional problems by using an aqueous solution comprising an iron compound, a barium compound and an alkaline substance. The present invention has been accomplished on the basis of this discovery.